In the United States commercial aircraft having greater than a 7500 pound payload and thirty passenger seat capacity are required by Federal Aviation Agency (FAA) regulations (Title 14 CFR "Aeronautics and Space", parts 0-199) to provide historical recording of certain mandatory flight parameters. The mandated flight parameters, which must be continuously recorded during the operational flight profile of the aircraft, include a minimum number of functional parameters considered essential for reconstructing the aircraft flight profile in post accident investigation proceedings. Present recording requirements specify a minimum 25 hour interval.
The data recording is made on a Flight Data Recorder (FDR) designed to withstand a crash environment. These FDRs are either of two types: (i) electromechanical or (ii) solid-state memory. At present the electromechanical recorders represent the majority used on both civil and military aircraft. They include both analog signal, metal foil and digital signal, magnetic tape. The digital signal recorders (solid-state or electromechanical) represent the contemporary standard for all new aircraft. This results from the development of high accuracy, fast response engine digital signal sensors, which have stimulated requirements for improved flight data monitoring systems. The digital recording system signal formats are defined by ARINC 717, which replaces the ARINC 573 definitions of analog signal formats for implementing the FAA performance specifications for historical recording of the flight parameters.
The recording system input data is, as is the remaining nonrecorded flight data, sensed within the various operating systems of the aircraft, acquired and conditioned in a digital flight data acquisition unit (DFDAU), and presented to the digital flight data recorder (DFDR) for preserved recording. The DFDAU is the collecting source for the flight data recorder as well as the other utilization equipment (e.g. airborne integrated data system, AIDS). The DFDR cannot function without the DFDAU. The DFDAU, in turn, receives the flight data from the multifarious sensor signal groups of the aircraft, including the Air Data Computer, Flight Management System, etc. As a consequence overall recording system integrity is dependent on the data sensors and sensor signal conditioning circuitry, the data acquisition unit, the flight data recorder, and the aircraft interconnecting wiring.
The extended nature of the components involved make reliability of the system a major concern. Prior art recording systems include built-in test equipment (BITE) for the DFDAU and DFDR, but not the sensors. The sensors are not subject to BITE testing due to practical constraints, e.g. nature of the sensor and/or the BITE requirements, or the existence of different manufacturer and suppliers of the equipment; manfacturers of the data acquisition and recorder hardware are not those which provide the sensors. As a consequence the sensor interface is untested during flight.
To assure recording system integrity the airlines are required by FAA (or other regulatory agency) to periodically certify operation of the flight data recording system on each aircraft. This requires that the DFDR be removed from the aircraft and tested on a scheduled basis; typically every 2,000 hours. The data stored in the DFDR is read from the recorder and transscribed to determine that all elements of the system are functional. The DFDR must then be routed through the airline maintenance cycle prior to being returned to service. This not only represents high cost, but the method of test (off-line) still allows the risk of overlooking overall system integrity, e.g. underestimating the significance or lack of significance of any given units of recorded data.